A radar device configured to detect a target has been known. The radar device is configured to transmit and receive frequency-modulated electromagnetic waves, thereby estimating a distance from the radar device to the target having reflected the electromagnetic waves, a speed of the target relative to the radar device and a target direction (angle) from the radar device.
The radar device is configured to receive the electromagnetic waves reflected on the target at a plurality of receiving antennas. The radar device can calculate the target direction (an arrival direction of the electromagnetic wave) by a phase difference of the reflected waves (reflected electromagnetic waves) received by the two adjacent antennas. The phase difference is generated due to a difference of path lengths (path length difference) from the target to the respective antennas.
When the two adjacent receiving antennas are distant from each other by a half wavelength or longer of the electromagnetic waves to be received, a case may occur in which a phase difference of reflected waves from the same target, which are to be received by the two receiving antennas, is 360° (2πrad) or greater in a detection region from −90° to +90°. Since sin(α)=sin(α+360°, the radar device cannot distinguish a phase difference α (−180°<α≤180°) from a phase difference α+360°, for example. For this reason, it is not possible to estimate the target direction as one only by the phase difference. Also, when an interval between the receiving antennas is narrowed, the phase difference is within a range from −180° to +180° but antenna characteristics are deteriorated.
FIG. 1 depicts an example of the phase difference when targets are at different directions from a radar device. In FIG. 1 a first receiving antenna and a second receiving antenna, which are receiving antennas of the radar device, are shown. An interval between the first receiving antenna and the second receiving antenna is d. At a left side of FIG. 1, the first receiving antenna and the second receiving antenna receive reflected waves from a target existing in a direction in which an angle to a front direction of the radar device is θ. The interval d between the antennas is greater than a wavelength of the reflected wave. At the left side of FIG. 1, it is assumed that a phase difference of the reflected waves to be received at the first receiving antenna and the second receiving antenna is α(−180<α≤180°). At a right side of FIG. 1, the first receiving antenna and the second receiving antenna, which are the same as the left side, receive reflected waves from a target existing in a direction in which the angle to the front direction of the radar device is ϕ (>θ). At the right side of FIG. 1, it is assumed that a phase difference of the reflected waves to be received at the first receiving antenna and the second receiving antenna is α+360°. Herein, since the radar device cannot distinguish the phase difference α from the phase difference α+360°, it is not possible to estimate whether the target direction is the direction of θ or ϕ only by the phase difference.
Therefore, a technology of transmitting electromagnetic waves from a plurality of antennas (for example, a first transmission antenna and a second transmission antenna) of a radar device and estimating a target direction on the basis of a target direction estimated from a phase difference and receiving levels with respect to the respective transmission antennas has been known. That is, when a target exists in a direction corresponding to the phase difference α and when a target exists in a direction corresponding to the phase difference α+360°, level differences between a received signal of a signal by the first transmission antenna and a received signal of a signal by the second transmission antenna are different. By using the corresponding characteristic, a receiving level difference is calculated with respect to an estimated direction, so that it is possible to determine whether the target direction is the direction corresponding to the phase difference α or the phase difference α+360°.
Patent Document 1: Japanese Patent Application Publication No. 2015-068724A
However, when a detection direction range of the target in the radar device is expanded, the receiving level at the receiving antenna is lowered in the vicinity of a boundary of the detection direction range. When the receiving level is lowered, the level difference is reduced, so that it is difficult to estimate the target direction by using the level difference. Also, since the plurality of transmission antennas is used so as to estimate the target direction with the receiving level difference, a configuration of the radar device becomes complicated.